Conventionally, a communication unit installed in a facility such as a toll booth of an expressway, a parking or the like and a communication apparatus for use in a vehicle have a wireless communication with each other through a well-known communication technique. In such a technique, the maximum number of data transmissions (i.e., the number of times of repeating data transmission) from the communication apparatus in the vehicle to the communication unit in the facility is determined. For example, Japanese patent document JP-A-H11-39523 (also available as U.S. patent document U.S. Pat. No. 5,952,940) discloses a technique as described above.
The communication apparatus in the above technique repeats data transmission for the maximum number of times if the acknowledgement of the data reception is not coming from the communication unit in the facility, and, after trying data transmission for the maximum number of times, the data transmission is terminated and the process proceeds to the next step.
The maximum number of data transmissions was a fixed number in the conventional technique. For example, an ETC apparatus in the vehicle for electronic toll collection has a fixed number of 127 (i.e., the maximum number of relevant standard) for repeating data transmission due to an importance of the data that is used for toll collection.
However, the fixed number of data transmission does not provide a flexibility for re-transmission of data from the communication apparatus in the vehicle. That is, for example, if changing the number of data re-transmission is not allowed, the communication apparatus has to perform all instances of communication by the same degree of efforts even when data importance varies data to data. In other words, important data may be transmitted for a greater number of times in comparison to the number of transmissions for the less important data, or the number of re-transmissions of the important data might be increased while decreasing the number of data re-transmissions of not-so-important data. Further, for a certain type of data such as probe data that is important because of its real-time characteristics, transmitting newly-generated data may be more beneficial than repeating re-transmission of old data in terms of keeping data freshness.